Implement Profile UIs using Dart Web UI.

Make datalist bindings available for new elements.
Tweak PacProfile to allow pacScript + pacUrl at the same time.
Add a PacProfile in the testing settings.
Fix a bug in the English translation.
Add a "Export PAC" test in options.html.

Remove code hack about dumping JSON in pac_gen_test.dart which I forgot to
remove after testing. Sorry.